NGC925

[alan4908]

Since I image from an almost dark site, I was keen to experiment with very long exposures (1800s) with my Lum filter in order to maximize detail of a faint (10.7 mag) galaxy, NGC925.  

After stacking the Lum subframes I noticed that the ADU of some of the star cores where in the non-linear region of my camera, which is not good. Previous measurements have shown me that my camera starts to go non-linear at just over 47k ADU, so I decided that a good strategy would be to reject the pixels above 45k and replace them with the values from a 600s Lum frame to create a High Dynamic Range image.  Due to the UK weather, I'm rather obsessed about imaging efficiency, so rather than shoot some additional 600s Lum subs, I decided to create a psuedo Lum 600s frame from my stack of RGB 600s subs. I then used CCDstack to create the Lum HDR image. The processed result is below which has yielded quite a deep image.  I've also included an annotation that has identified an additional six galaxies although the image appears to include many more. 

Alan

 

LIGHTS: L:18 x 1800s, R:9, G:9, B:9 x 600s. DARKS:30, BIAS:100, FLATS:40 all at -20C. 

[MartinB]

It is a lovely deep image and a lot of dedication has gone into the processing with great results.  9 hours of luminence well used!

I've never been convinced that using longer subs allows you to go deeper once background sky glow has started to build significantly.  What was your background ADU value on your 1800 sec subs?  Did you loose any subs due to cloud or other gremlins? 

[alan4908]

12 minutes ago, MartinB said:

It is a lovely deep image and a lot of dedication has gone into the processing with great results.  9 hours of luminence well used!

I've never been convinced that using longer subs allows you to go deeper once background sky glow has started to build significantly.  What was your background ADU value on your 1800 sec subs?  Did you loose any subs due to cloud or other gremlins? 

Hi Martin

Thanks for the comment 

I previously shared your skepticism on the skyglow effect which is why I've previously limited myself to 10min subs.

To answer your questions: on the 1800s I get an average background ADU of about 3500. Having looked at some 600s Lum subs, they are 3 times less so, I presume this is skyglow.  I did notice that I seem to be detecting some faint dust (or noise) but I didn't emphasize this in the processing. 

In terms of lost subs - yes, I lost about 3, all due to cloud. However, I think I would have lost these even if I had selected 10min subs, since if I look at these 1800s "cloudy subs" hardly any signal is present. Since going unguided and knowing that my cloud detector is not perfect, I currently have it configured such that my scope is allowed to image even when the detector believes it is partially cloudy rather than clear. Whilst this results in a few lost subs it does seem to increase my imaging throughput.

I would appreciate your thoughts on what you believe is an acceptable upper ADU background limit, I cannot quite figure this out - particularly since I get get around white clipping by creating a HDR image from the pseudo lum from my 600s RGB subs.

Alan